Brazilian officials began painstaking forensic work today to determine whether the human remains exhumed near here are those of Nazi war criminal Josef Mengele.

Brazil has insisted on conducting the investigation and has declined formal offers of assistance by experts from Israel, West Germany and the United States who arrived here today, according to Brazilian and West German officials.

Sao Paulo Federal Police Chief Romeu Tuma said dental and other medical records "with information on Mengele's bone structure, height and hair color have arrived from Germany."

He told reporters that experts at the city morgue "have begun cleaning the bones, after which they will start assembling the skeleton" of a man who used the name of an Austrian and died while swimming in the ocean in 1979.

Identification "could take a few hours or a few weeks, but due to the historical importance of the case, we will try to come up with the answer as soon as possible," Tuma said.

He added: "There are very strong indications that the remains do indeed belong to Mengele."

In New York, Nazi-hunter and concentration camp survivor Simon Wiesenthal said that he had changed his mind since Thursday, when he expressed doubt that the body of Mengele had been found, and that he is now "less skeptical," United Press International reported.

"I have been working on this since 7 a.m. yesterday before coming here from Austria late in the afternoon," he said. "What I have found out has conformed with statements coming from Brazil."

In West Germany, a prosecutor said today that police were treating the case seriously, but he cautioned about leaping to conclusions that Mengele was dead, Washington Post correspondent William Drozdiak reported from Bonn. Hans Eberhard Klein, the Frankfurt official in charge of the Mengele case since 1974, said at a press conference that there was scant data available to aid tests on the grave's remains, and only Mengele's height (about 5 feet 10 inches ) and his date of birth (March 16, 1911) were known for sure.

There was no reliable information about wounds, so it was unclear whether bone tests could prove helpful. Moreover, Klein said, Mengele's dental records sent to Sao Paulo date from 1938 and may be of limited value. "A lot can happen to a man's teeth in over 40 years, and we have no other positive means of identification of the skeleton," he said.

Klein emphasized that until Mengele's death had been verified and all doubts laid to rest, the global search for him would continue.

He said that letters found in the home of a former employe of Mengele's family firm provided the crucial link to the Brazilian grave that may have contained the remains of Mengele, known as the "Angel of Death" for his ghoulish experiments on twins and children at the Auschwitz concentration camp.

While investigators check the physical evidence, which is expected to take several days, aspects of testimony by an Austrian couple who claim to have sheltered Mengele are being cross-checked.

The couple, Wolfram and Liselotte Bossert, said that Mengele's son Rolf visited Brazil in 1977 and again in 1979, after the Nazi was said to have drowned in a swimming accident at Bertioga Beach near Sao Paulo.

At a press conference today, Police Chief Tuma showed a family photograph that he said had been sent to the Bosserts' house around Christmas 1983, showing Rolf Mengele and his wife.

Brazilian police records and eyewitnesses also confirm the couple's account of a Feb. 7, 1979, drowning at Bertioga of a man identified as Wolfgang Gerhard.

While the true identity of the man buried at Embu on Feb. 8, 1979, under the name of Wolfgang Gerhard and exhumed yesterday remains a mystery, German police confirm that a Wolfgang Gerhard also is buried in the Austrian town of Graz, where he died in an auto accident in 1978.

The Bosserts also claim that after his arrival in Brazil around 1969, Mengele used the identity of Peter Gerhard and together with several other northern Europeans lived at Caieiras near Sao Paulo on the farm of Janus Stammer, an Austrian. A Brazilian newspaper quoted the Bosserts as saying that Mengele argued with Stammer and was obliged to leave the farm.

The Bosserts claim that in 1976 Mengele assumed the identity of Wolfgang Gerhard, who had returned to Europe, leaving his documents behind. In March 1977, they say, Mengele went to live at their house at Estrada do Alvarenga.

Diplomatic sources said Stammer's wife had been questioned informally by German and Brazilian officials. She confirmed knowing a Peter Gerhard and said that on occasion she had received mail for him at her post office box, the sources said.

Brazilian press reports quoted the Bosserts as saying it was Stammer who made monthly deliveries of money of German origin to Mengele after he moved to their house.

Police Chief Tuma said, "The information we have is that a self-protection organization does exist among those sought for crimes against humanity." But no official information was available to suggest that Stammer was a member.

Officials at the Medical Legal Institute in Sao Paulo said they would provide regular information for the press starting Monday, but forensic tests were expected to take up to 10 days. The institute's vice director, Jose Antonio de Mello, found seven teeth yesterday when the skull was exhumed. He found two dental bridges or prostheses of recent workmanship, as well as a gold filling that could date to the period covered by medical records sent to the West German Embassy in Brasilia and forwarded to the institute.

Washington Post correspondent Drozdiak also reported the following:

West German prosecutor Klein said at a press conference that the letters that provided a link to the grave in Brazil were found when police searched the home of an unidentified man in Neu-Ulm, near Stuttgart, after learning from a university professor that the man boasted about arranging financial aid for the Nazi fugitive. The man worked for the Mengele family's farm machinery business in the Bavarian town of Guenzburg.

Klein said the police discovered a cache of letters in the wardrobe of the man's wife on May 31. Two of the letters, alluding to Mengele's death, were traced to the Bosserts in Sao Paulo.

At least seven other letters, dated between 1972 and 1978, may have been sent by Mengele himself, according to Klein. He said that the missives did not cite any names but that police cryptologists quickly recognized that the initials and information in the letters referred to Mengele and his life on the run.